What, When and Who, this is #1 rule regarding Auditing, and it is no difference in the world of network security. We want to know what was accessed, by who and when was it done.
Netwrix have a suite of tools to audit changes on your network. This can range from Active Directory objects , File servers , Sharepoint Servers, VMware , SQL Databases just to name a few.
For a full list of solutions
I implemented this system a few months back to show transparency regarding the access our team have and to make sure no boundaries were not being overstepped. The setup is not as straight forward but it was worth it and the support was decent.
NB: Be aware it would be wise to create a separate GPO for the servers you want audited and either have them all in one OU container or create a security group for them and apply a GPO security filter.
So if auditing your network and systems are on your 2015 to do list I suggest looking at this product.